3d memory device

ABSTRACT

A 3D memory device comprising: a substrate; at least one first group of at least three first U-shaped memory cells strings each including a first buried string portion, a first source line selector side string portion and a first bit line selector side string portion, wherein the first buried string portion is formed in the substrate and connects the first source line selector side string portion and the first bit line selector side string portion, each of the first U-shaped memory cells strings including memory cells stacks along the first source line selector side string portion and along the first bit line selector side string portion; and at least one second group of at least three second U-shaped memory cells strings each including a second buried string portion, a second source line selector side string portion and a second bit line selector side string portion. The second buried string portion is formed in the substrate and connects the second source line selector side string portion and the second bit line selector side string portion, each of the second U-shaped memory cells strings including memory cells stacks along the second source line selector side string portion and along the second bit line selector side string portion. The first and second source line selector side string portions are between the first and second bit line selector side string portions. The at least three first U-shaped memory cells strings are mutually co-planar and one surrounded by the other, and the at least three second U-shaped memory cells strings are mutually co-planar and one surrounded by the other.

BACKGROUND Technical Field

The present invention generally relates to the field of semiconductordevices and in particular to the field of semiconductor memories. Morespecifically, the present invention relates to the so-called“three-dimensional” (“3D”) semiconductor memory sector.

Related Art

In the industry of non-volatile semiconductor memory (memory devicesthat can hold the data stored therein even in the absence of an energysource), 3D semiconductor memories (“3D memories”) represent anevolution of traditional “two-dimensional” memories semiconductor (“2D”memories, in which memory cells are formed as a single layer on asubstrate of semiconductor material), which allows to exceed the limitsof the 2D structure to further increase the integration scale and,therefore, further increase data storage capacity per unit area.

Examples of non-volatile 3D semiconductor memory devices with NANDarchitecture (in which there are serially connected memory cell groupsto form memory cell strings) are described in US 2012/300547 A1, US2015/0155371 A1, US 2013/0153978 A1, US 2015/0017771 A1.

In particular, in US 2015/017771 A1 there are described some memory cellblock structures (“memory blocks”), where by memory block it is meantthe structural unit which, replicated in two dimensions, constitutes amemory cell array (memory matrix).

SUMMARY OF THE INVENTION

The Applicant noticed that known memory block architectures, such asthose described in US 2015/017771 A1, have a margin of improvement interms of compactness, resulting in improved data storage capacity perunit area.

An object of the present invention is to propose a memory blockarchitecture for a more compact 3D semiconductor memory than the knownarchitectures, enabling further increases in data storage capacity perunit area.

According to with the present invention, it is proposed a 3D memorydevice comprising:

-   -   a substrate;    -   at least one first group of at least three first “U”-shaped        strings of memory cells each including a first buried string        portion, a first source line selector side string portion and a        first bit line selector side string portion, wherein the first        buried string portion is formed in the substrate and connects        the first source line selector side string portion and the first        bit line selector side string portion, each of the first        “U”-shaped string of memory cells including stacks of memory        cells along the first source line selector string side portion        and along the first bit line selector side string portion; and    -   at least one second group of at least three second “U”-shaped        strings of memory cells each including a second buried string        portion, a second source line selector side string portion and a        second bit line selector side string portion, wherein the second        buried string portion is formed in the substrate and connects        the second source line selector side string portion and the        second bit line selector side string portion, each of the second        “U”-shaped string of memory cells including stacks of memory        cells along the second source line selector side string portion        and along the second bit line selector side string portion.

The first and second source line selector side string portions arebetween the first and second bit line selector side string portion.

The at least three first “U”-shaped memory cells strings are mutuallyco-planar and one surrounded by the other, and the at least three second“U”-shaped memory cells strings are mutually co-planar and onesurrounded by the other.

Preferably, the at least three first “U”-shaped memory cells stringshave the respective first buried string portions formed at a first, asecond and a third different depths in the substrate and with a first, asecond and a third different lengths, the at least three second“U”-shaped memory cells strings have the respective second buried stringportions formed at the first, the second and the third depths in thesubstrate and with the first, the second and the third lengths.

Advantageously, the length of the first and second buried stringportions increases with their depth in the substrate.

Preferably, the first depth is less than the second depth and the seconddepth is less than the third depth. Preferably, the first length is lessthan the second length and the second length is less than the thirdlength.

Advantageously, the at least three first “U”-shaped memory cells stringsare co-planar with the at least three second “U”-shaped memory cellsstrings.

In embodiments, the 3D memory device may further comprise:

-   -   a plurality of source line selector side word lines stacked over        the substrate, wherein the plurality of source line selector        side word lines surrounds the first source line selector side        string portions and the second source line selector side string        portions; and    -   a first plurality and a second plurality of bit line selector        side word lines stacked over the substrate, wherein the first        plurality of bit line selector side word lines surrounds the        first bit line selector side string portions and the second        plurality of bit line selector side word lines surrounds the        second bit line selector side string portions.

In embodiments, the 3D memory device may also comprise:

-   -   source line selectors stacked over the plurality of source line        selector side word lines, wherein the source line selectors        surround the first source line selector side string portions and        the second source line selector side string portions switch,        which source line selectors include a source line selector for        each of the first and second source line selector side string        portions; and    -   first bit line selectors stacked over the first plurality of bit        line selector side word lines and second bit line selectors        stacked above the second plurality of bit line selector side        word lines, wherein the first bit line selectors surround the        first bit line selector side string portions and the second bit        line selectors surround the second bit line selector side string        portions, and wherein the first bit line selectors comprise a        first bit line selector for each of the first bit line selector        side string portions and the second bit line selectors comprise        a second bit line selector for each of the second bit line        selector side string portions.

The plurality of source line selector side word lines can include astack of word line layers on the source line selector side, the sourceline selector side word lines in each layer being electricallyconnected, and the source line selectors may be formed so that they arecontrolled by a single common source line selector control signal.

In embodiments, the first bit line selectors may be formed so as to becontrolled by a single common first bit line selector control signal,and the second bit line selectors may be formed so as to be controlledby a single common second bit line selector control signal.

The 3D memory device may comprise at least one first bit lineoperatively associated to at least two of the at least three first andsecond “U”-shaped memory cells strings. The at least one first bit linemay be connected, through a respective first bit line selector andsecond bit line selector, to the first bit line selector side stringportions of the at least two of the at least three first “U”-shapedmemory cells strings and to the second bit line selector side stringportions of the at least two of the at least three second “U”-shapedmemory cells strings.

Preferably, the center lines of the first buried string portions areplaced along a same respective line, and the center lines of the secondburied string portions are placed along a same respective line.

Thanks to the present invention, it is possible to realize 3Dsemiconductor memories with data storage capacity improved over thoseknown in the art.

BRIEF PRESENTATION OF THE ATTACHED FIGURES

These and other features and advantages of the present invention willbecome more apparent by reading the following detailed description ofsome of its exemplary embodiments being, to be considered asnon-limitative. For a better intelligibility, the following descriptionwill make reference to the attached figures, briefly presented below:

FIGS. 1A and 1B show the constituent basic elements of an 8 memory cells“U” shaped string;

FIG. 1C shows the equivalent electrical circuit of the string of memorycells of FIGS. 1A and 1B;

FIG. 1D shows the equivalent electrical circuit of a string of memorycells similar to that of FIGS. 1A and 1B but comprising 96 memory cells;

FIG. 2 is an isometric view of a portion of a memory device according toa first exemplary embodiment of the invention;

FIG. 3 corresponds to FIG. 2 but with a portion of substrate representedin transparency;

FIG. 4 is a top plan view of the portion of memory device of FIG. 2;

FIG. 5 corresponds to FIG. 4 but with the bit lines and the source linerepresented in transparency;

FIG. 6 is a top plan view of the portion of memory device of FIG. 2 atthe level of the top surface of the substrate, with the substraterepresented in transparency as in FIG. 3;

FIG. 7 is a plan view from below of the memory device portion of FIG. 2with the substrate represented in transparency as in FIG. 3;

FIG. 8 is a front view (in the x direction shown in the figures) of thememory device portion of FIG. 2 with the substrate represented intransparency as in FIG. 3;

FIG. 9 is a side view (in the y direction shown in the figures) of thememory device portion of FIG. 2 with the substrate represented intransparency as in FIG. 3;

FIG. 10 is an equivalent circuit diagram of a portion of a memory devicehaving the architecture shown in FIGS. 2-9 but with a greater number ofmemory cells (96 instead of 8);

FIGS. 11-16 are sectional views of the memory device portion of FIG. 2,according to the sectional plans XI, XII, XIII, XIV, XV, XVI indicatedin FIG. 5;

FIG. 17 is a sectional view of the memory device portion of FIG. 2,according to the plane XVII-XVII shown in FIG. 5;

FIG. 18 is a front view (in the y direction shown in the figures) ofFIG. 17;

FIG. 19 is an isometric view of a portion of a memory device inaccordance with a second exemplary embodiment of the invention;

FIG. 20 corresponds to FIG. 19 but with a portion of substraterepresented in transparency;

FIG. 21 is a top plan view of the memory device portion of FIG. 19;

FIG. 22 corresponds to FIG. 21 but with the bit lines and the sourceline represented in transparency;

FIG. 23 is a top plan view of the memory device portion of FIG. 19 atthe level of the top surface of the substrate, with the substrate asrepresented in transparency in FIG. 20;

FIG. 24 is a plan view from below of the memory device portion of FIG.19 with the substrate as represented in transparency as in FIG. 20;

FIG. 25 is a front view (in the x direction shown in the figures) of thememory device portion of FIG. 19 with the substrate represented intransparency as in FIG. 20;

FIG. 26 is a side view (in the y direction shown in the figures) of thememory device portion of FIG. 19 with the substrate represented intransparency as in FIG. 20;

FIG. 27 is an equivalent circuit diagram of a portion of a memory devicehaving the architecture shown in FIGS. 19-26 but with a greater numberof memory cells (96 instead of 8);

FIGS. 28 and 29 are sectional views of the memory device portion of FIG.19, according to the sectional planes XXVIII and XXIX shown in FIG. 22;

FIG. 30 is a sectional view of the memory device portion of FIG. 19,according to the plane XXX indicated in FIG. 22;

FIG. 31 is a front view (in the y direction shown in the figures) ofFIG. 30;

FIG. 32 is an isometric view of a portion of a memory device inaccordance with a third exemplary embodiment of the invention;

FIG. 33 corresponds to FIG. 32 but with a portion of the substraterepresented in transparency;

FIG. 34 is a top plan view of the portion of memory device of FIG. 32;

FIG. 35 corresponds to FIG. 34 but with the bit lines and the sourceline represented in transparency;

FIG. 36 corresponds to FIG. 35 but with bit line contact platesrepresented in transparency;

FIG. 37 is a top plan view of the memory device portion of FIG. 32 atthe level of the top surface of the substrate, with the substraterepresented in transparency as in FIG. 33;

FIG. 38 is a plan view from below of the memory device portion of FIG.32 with the substrate represented in transparency as in FIG. 33;

FIG. 39 is a front view (in the x direction shown in the figures) of thememory device portion of FIG. 32 with the substrate represented intransparency as in FIG. 33;

FIG. 40 is a side view (in the y direction shown in the figures) of thememory device portion of FIG. 32 with the substrate represented intransparency as in FIG. 33;

FIG. 41 is an equivalent circuit diagram of a portion of a memory devicehaving the architecture shown in FIGS. 32-40, but with a greater numberof memory cells (96 instead of 8);

FIGS. 42 and 43 are sectional views of the memory device portion of FIG.32, according to the sectional planes XLII and XLIII indicated in FIG.36;

FIG. 44 is a front view (in the y direction shown in the figures) ofFIG. 43;

FIG. 45 is an isometric view of a portion of a memory device inaccordance with a fourth exemplary embodiment of the invention;

FIG. 46 corresponds to FIG. 45 but with a portion of substraterepresented in transparency;

FIG. 47 is a top plan view of the memory device portion of FIG. 45;

FIG. 48 corresponds to FIG. 47 but with the bit lines and the sourceline represented in transparency;

FIG. 49 is a top plan view of the memory device portion of FIG. 45 atthe level of the top surface of the substrate, with the substraterepresented in transparency as in FIG. 46;

FIG. 50 is a plan view from below of the memory device portion of FIG.45 with the substrate represented in transparency as in FIG. 46;

FIG. 51 is a front view (in the x direction shown in the figures) of thememory device portion of FIG. 45 with the substrate represented intransparency as in FIG. 46;

FIG. 52 is a side view (in the y direction shown in the figures) of thememory device portion of FIG. 45 with the substrate represented intransparency as in FIG. 46;

FIG. 53 is an equivalent circuit diagram of a portion of a memory devicehaving the architecture shown in FIGS. 45-52 but with a greater numberof memory cells (96 instead of 8);

FIGS. 54 and 55 are sectional views of the memory device portion of FIG.45, according to the sectional plans LIV-LIV and LV-LV indicated in FIG.48;

FIG. 56 is a front view (in the y direction shown in the figures) ofFIG. 55;

FIG. 57 is an isometric view of a memory device portion in accordancewith a fifth exemplary embodiment of the invention;

FIG. 58 corresponds to FIG. 57 but with a portion of substraterepresented in transparency;

FIG. 59 is a top plan view of the memory device portion of FIG. 57;

FIG. 60 corresponds to FIG. 59 but with the bit lines and the sourceline represented in transparency;

FIG. 61 is a top plan view of the memory device portion of FIG. 45 atthe level of the top surface of the substrate, with the substraterepresented in transparency as in FIG. 58;

FIG. 62 is a plan view from below of the memory device portion of FIG.57 with the substrate represented in transparency as in FIG. 58;

FIG. 63 is a front view (in the x direction shown in the figures) of thememory device portion of FIG. 57 with the substrate represented intransparency as in FIG. 58;

FIG. 64 is a side view (in the y direction shown in the figures) of thememory device portion of FIG. 57 with the substrate represented intransparency as in FIG. 58;

FIG. 65 is an equivalent circuit diagram of a portion of a memory devicehaving the architecture shown in FIGS. 57-65 but with a greater numberof memory cells (96 instead of 8);

FIGS. 66 and 67 are sectional views of the memory device of FIG. 57portion, according to the sectional plans LXVI-LXVI and LXVII-LXVIIshown in FIG. 60, and

FIG. 68 is a front view (in the y direction shown in the figures) ofFIG. 67.

DETAILED DESCRIPTION OF EMBODIMENT EMBODIMENTS OF THE INVENTION

In FIGS. 1A and 1B there is schematically shown the structure of a“U”-shaped string of 8 memory cells of a 3D non-volatile NANDsemiconductor memory, with the constituent basic elements; the structureis per-se known and will not be described in detail.

Reference numeral BL indicates a bit line, made of electricallyconductive material, typically metal. Reference numeral SL indicates asource line, also made of electrically conductive material, for examplemetal. A bit line selector BLS is electrically connected to the bit lineBL via a bit line contact BLC; the bit line selector BLS for examplecomprises a transistor (eg. a MOS transistor) for selectivelyelectrically connecting/disconnecting the “U”-shaped string of memorycells to the bit line BL. A source line selector SLS is electricallyconnected to the source line SL; the source line selector for examplecomprises a transistor (eg. a MOS transistor) for selectivelyelectrically connecting/disconnecting the “U”-shaped string of memorycells to the source line SL. Reference numerals CG0-CG7 indicate 8control gates of a same number of memory cells (indicated by C0-C7 inFIG. 1C), arranged along a tubular structure with a “U” shape; inparticular, the tubular structure comprises a bit line selector sidestring portion (or pillar) P-BL and a source line selector side stringportion (or pillar) P-SL, the two pillars P-BL and P-SL being joined bya lower tubular portion USH (according to the orientation of thefigures), or buried string portion, which electrically connects the twopillars P-BL and P-SL (the lower tubular portion USH is typically formedin a substrate, not shown in FIGS. 1A and 1B, which may be a layerreferred to as “pipe gate layer”, of the same material as the controlgate, for example polysilicon, or a layer of a semiconductor material,such as silicon, or in an insulating material such as silicon oxide).

The bit line selector side pillar P-BL and the source line selector sidepillar P-SL comprise various layers of material, not shown in thefigures, that define the structure of the memory cells. In particular,the memory cells may be of a Charge Trap (CT) type or a floating gatetype (Floating Gate or FG), for example. The specific structure of thememory cells is not essential for the purposes of the present invention,which is for example applicable to both CT memory cells and FG memorycells, as well as to memory cells of different structure.

FIG. 1C shows the equivalent electrical circuit of the “U”-shaped stringof memory cells of FIGS. 1A and 1B. The bit line selector BLS iscontrolled by a bit line selection signal #BLS, the source line selectorSLS is controlled by a source line selection signal #SLS, and the 8control gates CG0-CG7 of the memory cells C0-C7 are controlled byrespective control gate (selection) signals CG0 #-# CG7. The bit lineselection signals #BLS, the source line selection signals #SLS and thecontrol gate selection signals CG0 #-# CG7 are generated by decodingcircuits of memory cell addressing signals. The control gates CG0-CG7constitute the word lines of the memory. In particular, the controlgates CG0-CG3 are source line selector side word lines and the wordlines CG4-CG7 are bit line selector side word lines.

In the “U”-shaped memory cells string of FIGS. 1A and 1B, 4 controlgates CG0-CG3 (and thus 4 memory cells) are arranged in succession,stacked along the source line SL selector side pillar and 4 controlgates CG4-CG7 (and thus 4 more memory cells) are arranged in succession,stacked along the bit line BL selector side pillar. Each of the 4control gates CG0-CG3 along the source line selector side pillar P-SL isat the same height (according to the direction indicated by z in thefigures) of a respective one of the 4 control gates CG4-CG7 arrangedalong the bit line selector side pillar P-BL, and for this reason the“U”-shaped string is said to have 4 layers. The state of the art knows3D memory devices with 24, 36 and even 48 layers, each “U”-shaped stringof memory cells containing 48, 72 and 96 memory cells, respectively.FIG. 1D shows the equivalent electrical circuit of a “U”-shaped stringof memory cells with 48 layers, with 96 memory cells, of which 48 memorycells C0-C47 (control gates CG0-CG47, controlled by respective controlgate selection signals #CG0-#CG47) are arranged in succession, stackedalong the source line selector side pillar P-SL, and other 48 memorycells C48-C95 (control gates CG48-CG95, controlled by respective controlgate selection signals #CG48-#CG95) are arranged in succession, stackedalong the bit line selector side pillar P-BL. The present invention isapplicable irrespective of the number of layers (i.e. irrespective ofthe number of memory cells in the strings).

First Embodiment

A first exemplary embodiment of the invention is shown in FIGS. 2-18.Similarly to FIGS. 1A and 1B, the figures (except for FIG. 10, whichshows the equivalent electrical circuit of a memory with 48 layers)illustrate for simplicity a memory with 4 layers, but this is not to beconstrued as limiting. In particular, FIGS. 2-18 relate to a so-calledmemory (cell) block, to be understood as a set of memory cells thatshare the same set of (eight in the example considered) control gatesCG0-CG7.

The memory block includes two groups (first and second group) of 16“U”-shaped strings of memory cells, each “U”-shaped string having thestructure shown in FIGS. 1A and 1B. In a first group of 16 “U”-shapedstrings of memory cells (at the left of the figures) each “U”-shapedstring is electrically connectable/disconnectable to/from a respectivebit line among the 16 bit lines BL0-BL15 through a respective bit lineselector (MOS transistor), wherein the 16 bit line selectors of the 16“U”-shaped strings of the first group are indicated in their entiretywith BLS0 and are controlled by a same bit line selection control signal#BLS0. Similarly, in the second group of 16 “U”-shaped strings of memorycells (at the right in the figures) each “U”-shaped string iselectrically connectable/disconnectable to/from a respective bit lineamong the 16 bit lines BL0-BL15 through a respective bit line selector(MOS transistor), where the 16 bit line selectors of the first group of16 strings are indicated in their entirety with BLS1 and are controlledby a same bit line selection control signal #BLS1. Each “U”-shapedstring, of the first or of the second group, is electricallyconnectable/disconnectable to/from a source line SL common to (sharedby) all the memory blocks, by means of a respective source lineselector, where the source line 32 selectors of the 16 “U”-shapedstrings of the first group and of the 16 “U”-shaped strings of thesecond group are indicated in their entirety with SLS0 and arecontrolled by a same source line selection control signal #SLS0. Thesource line selectors SLS0 (and the source line SL) are arranged betweenthe bit line selectors BLS0 of the “U”-shaped strings of the first groupand the bit line selectors BLS1 of the “U”-shaped strings of the secondgroup. From the constructive point of view, the source line SL, thesource line selectors SLS0 and the control gates along the source lineside pillars P-SL are separated (electrically and physically) by the bitline selectors BLS0 and BLS1 and from the bit line side control gatesCG4-CG7 of the two groups of “U”-shaped strings by two excavations ortrenches T, in jargon called “slits” (cuts, cracks, separations,subdivisions) which extend in depth down to the top (according to theorientation of the figures) surface of the substrate S in which thelower tubular portions USHL0-USHL15 and USHR0-USHR15 of the “U”-shapedtubular structures are formed.

In each of the two groups of “U”-shaped strings, the “U”-shaped stringsare arranged in rows extending along the direction x; in the exampleconsidered here, the 16 “U”-shaped strings of each group are arranged in4 rows in the direction x. In each row, the (e.g. 4) “U”-shaped stringsare equally spaced by one pitch p. “U”-shaped strings (belonging torows) consecutive in the direction y are staggered by half a pitch p, inone direction and the other along the direction x, at zig-zag, so thatgroups of (2 in the example) of “U”-shaped strings belonging toalternated rows are aligned, lying on the same planes parallel to thedirection y (planes yz).

On top of each alignment of “U”-shaped strings along the direction y, apair of bit lines of the 16 bit lines BL0-BL15 extends. The bit linecontacts BLC to the bit line side pillars of the “U”-shaped strings thatare aligned along the direction y are staggered in one direction and theother along the direction x, resulting arranged at zig-zag, with respectto the centerline of such “U”-shaped strings. In this way it is possibleto realize the contact to the bit line side pillars of the “U”-shapedstrings that are aligned along the direction y to one or to the otherbit line of the pair that overhangs, alternately along the direction y.Preferably, between the bit line contacts BLC and the respective bitline side pillars it is possible to introduce an intermediate level ofcontact PC (pillar contact) to facilitate the achievement of theelectrical contact.

The source line contact SLC to the source line side pillars of the“U”-shaped strings are made substantially at the center of the sourceline pillars themselves.

In each of the two groups of “U”-shaped strings, the “U”-shaped stringsare made so as to have one of four possible depths (depth along thedirection z of the lower tubular portions USHL0-USHL3, USHR0-USHR3) andone of four possible widths (dimension along the direction y), whereinthe choice of the depth and the width of a given “U”-shaped string amongthe 16 of the own group is carried out depending on the position (alongthe direction x and along the direction y) of the “U”-shaped stringitself.

In particular, considering the “U”-shaped strings of the genericalignment along the direction y, the “U”-shaped string whose bit lineselector side pillar is more outward with respect to the centerline ofthe memory block (outer “U”-shaped string) has a greater width than the“U”-shaped string whose bit line selector side pillar is more internalwith respect to the centerline of the memory block (inner “U”-shapedstring), and the external “U”-shaped string has a greater depth than theinner “U”-shaped string (see for example in FIG. 6, the pairs of lowertubular portions USHL0 and USHL1, USHL2 and USHL3, USHL12 and USHL13,USHL14 and USHL15 for the group of “U”-shaped strings on the left, andthe pairs of lower tubular portions USHR1 and USHR0, USHR3 and USHR2,USHR15 and USHR14 for the group of “U”-shaped strings on the right). Inthis way, the internal “U”-shaped string is contained, both in thedirection y and in the direction z, within the external “U”-shapedstring.

In addition, considering, within the same group of 16 “U”-shapedstrings, a generic pair of adjacent alignments of “U”-shaped stringsalong the direction y, consecutive in the direction x, the depth of theinternal “U”-shaped string of a first alignment of said pair ofalignments is less than the depth of the internal “U”-shaped string ofthe second alignment of the pair, adjacent to the first alignment, andthe depth of the external “U”-shaped string of the first alignment ofthe pair is less than the depth the external “U”-shaped string of thesecond alignment of the pair, where the internal “U”-shaped strings havehowever depth less than that of the external “U”-shaped strings.

Advantageously, considering two pairs of “U”-shaped strings alignedalong the direction y, a first pair of “U”-shaped strings belonging toone of two groups of 16 “U”-shaped strings and a second pair belongingto the other of the two groups of 16 “U”-shaped strings, the width ofthe inner “U”-shaped string of the first pair is smaller than the widthof the inner “U”-shaped string of the second pair, and the width of theexternal “U”-shaped string of the first pair is smaller the width of theexternal “U”-shaped string of the second pair, where the inner“U”-shaped strings however have smaller width than the external“U”-shaped strings. As shown in FIG. 6, considering consecutivealignments along the direction y (in the direction x) the role of “firstpair” and “second pair” is alternately assumed by a pair of “U”-shapedstrings of the first group (left) and by a corresponding pair of“U”-shaped strings of the second group (right).

In this way it is possible to compact the “U”-shaped strings in thedirection x and in the direction y without the risk that two or morestrings come into mutual contact.

Advantageously, in each of the two groups of “U”-shaped strings, the“U”-shaped strings are made so as to have their respective centerlineson a same plane parallel to the direction x.

With the architecture of FIGS. 2-18, in which in particular it isnecessary to provide only two separations (slits) T, it is possible torealize very compact memory blocks containing a number of cells equal tothe number of layers multiplied by 64, with 16 bit lines and a sourceline. For comparison, considering the architecture of FIG. 5A of US2015/017771 A1, with an equal number of layers, in order to have anumber of cells equal to those of the memory block according to thefirst embodiment of the present invention, it should be necessary toplace side by side, in the direction y, two structures as those of FIG.5A of US 2015/017771 A1, but the memory block that would be obtained inthis way would be less compact, since it would be necessary to have fiveseparations instead of only two separations T. The savings in terms ofarea obtainable with the memory block architecture according to thefirst embodiment of the present invention is estimated to be a 10-15%.

Second Embodiment

A second exemplary embodiment of the invention is shown in FIGS. 19-31.As before, the figures (except for FIG. 27, which shows the equivalentelectrical circuit of a memory with 48 layers) illustrate for simplicitya memory with 4 layers, but this is not to be construed as limiting. Inparticular, as for the first embodiment described, FIGS. 19-31 relate toa memory (cell) block, i.e., the set of memory cells that share the sameset of (eight in the example considered) control gates CG0-CG7.

Compared to the first embodiment, in the second embodiment the 16 bitline selectors of the 16 “U”-shaped strings of the first group aregrouped into two groups of bit line selectors indicated in theirentirety with BLS0′ and BLS1′, respectively, and all the bit lineselectors of the group BLS0′, respectively BLS1′, are controlled by asame bit line selection control signal #BLS0′, respectively #BLS1′.Similarly, the 16 bit line selectors of the 16 “U”-shaped strings of thesecond group are grouped into two groups of bit line selectors indicatedin their entirety with BLS2′ and BLS3′, respectively, and all the bitline selectors of the group BLS2′, respectively BLS3′, are controlled bya same bit line selection control signal #BLS2′, respectively #BLS3′.

From the constructive point of view, in addition to the two separationsT already provided in the first embodiment, two additional separationsT1 are provided, extending in depth down to the topmost control gatelayer, to separate, electrically and physically, the bit line selectorsgroup BLS0′ from the bit line selectors group BLS1′, and the bit lineselectors group BLS2′ from the bit line selectors group BLS3′.

Having divided the bit line selectors into four groups, instead of intotwo groups as in the first embodiment, the number of bit lines can behalved (as the bit line selection signals are doubled): with the samenumber of “U”-shaped strings of the memory block, in the secondembodiment there are provided 8 bit lines BL0-BL7 instead of the sixteenbit lines of the first embodiment.

Thanks to the halving of the number of bit lines, in the secondembodiment it is possible to avoid having to provide the offset of thebit line contacts to the bit line side pillars of the “U”-shapedstrings. In fact, having halved the number of bit lines, on top of eachalignment of “U”-shaped strings along the direction y now only one bitline of the 8 bit lines BL0-BL7 extends.

The arrangement of the 16 “U”-shaped strings in each of the two groupsof “U”-shaped strings does not change with respect to the firstembodiment.

In each of the two groups of “U”-shaped strings, the first two rows of“U”-shaped strings along the direction x (that is, the two more externalrows of “U”-shaped strings with respect to the centerline of the memoryblock) are afferent to a the same group of bit line selectors BLS0′,BLS3′; the latter two rows of “U”-shaped strings along the direction x(that is, the two more internal rows of “U”-shaped strings with respectto the centerline of the memory block) are afferent to a same group ofbit line selectors BLS1′, BLS2′.

In FIGS. 19-31 the lower tubular segments of the 8 “U”-shaped strings ofthe “U”-shaped strings of the first two rows in the direction x of the“U”-shaped strings group on the left are indicated with USHL0 a-USHL7 a,and the lower tubular segments of the 8 “U”-shaped strings of the“U”-shaped strings of the second two rows in the direction x of thegroup of “U”-shaped strings on the left are indicated with USHL0 b-USHL7b; similarly, the lower tubular segments of the 8 “U”-shaped strings ofthe “U”-shaped strings of the first two rows in the direction x of thegroup of “U”-shaped strings on the right are indicated with USHR0a-USHR7 a, and the lower tubular segments of the 8 “U”-shaped strings ofthe “U”-shaped strings of the second two rows in the direction x of thegroup of “U”-shaped strings on the right are indicated with USHR0b-USHR7 b.

The architecture of the memory block in accordance with the secondembodiment is slightly less compact than the architecture according tothe first embodiment (in particular as a result of provision of fourslits T and T1), but has the advantage of a greater simplicity, evenconstructive, thanks to the halving of the number of bit lines, whichalso makes unnecessary the offset of the bit line contacts to the bitline side pillars of the “U”-shaped strings. The memory blockarchitecture in accordance with the second embodiment is stilladvantageous with respect to the architecture of FIG. 5A of US2015/017771 A1.

Third Embodiment

A third exemplary embodiment of the invention is shown in FIGS. 32-44.As before, the figures (except for FIG. 41, which shows the equivalentelectrical circuit of a memory with 48 layers) illustrate for simplicitya memory with 4 layers, but this is not to be construed as limiting. Inparticular, as for the previously described embodiments, FIGS. 32-44relate to a memory (cell) block, i.e., the set of memory cells thatshare the same set of (eight in the example considered) control gatesCG0-CG7.

The third embodiment can be considered a variation of the secondembodiment.

As in the second embodiment (and also as in the first embodiment), ineach of the two groups of “U”-shaped strings, the “U”-shaped strings arearranged in rows that extend along the direction x (in the exampleconsidered here, the 16 “U”-shaped strings of each group are arranged in4 rows in the direction x), and in each row the (4 in the example)“U”-shaped strings are equally spaced by one pitch p.

However, unlike the second embodiment (and also unlike the firstembodiment), the architecture of the memory block in accordance with thethird embodiment does not provide that the “U”-shaped strings (belongingto rows) consecutive in direction y are offset by half a pitch p, in onedirection and the other along the direction x, i.e. the zig-zagarrangement of the first two embodiments is not provided. Therefore, the“U”-shaped strings belonging to the (4 in the example) rows of eachgroup of 16 “U”-shaped strings are aligned, lying on the same planesparallel to the direction y (planes yz).

Consequently, although in the third embodiment the same number of (8 inthe example) bit lines BL0-BL7 as in the second embodiment is provided,it is again necessary (as in the first embodiment) to provide thestaggering of the bit line contacts to the bit line side pillars of the“U”-shaped strings. In fact, each alignment of “U”-shaped strings alongthe direction y is partially overhung by a pair of bit lines of the8-bit lines BL0-BL7.

In accordance with the third embodiment, the offset of the bit linecontacts to the bit line side pillars of the “U”-shaped strings isobtained with the aid of bit line contact plates P-BLC. The bit linecontact plates P-BLC are plates of electrically conductive material, forexample obtained starting from the same metal layer with which thesource line SL is made, generally rectangular in shape and substantiallycentered each one above a respective bit line selector side pillar ofthe “U”-shaped strings. The bit line contact plates P-BLC have dimensionin the direction y substantially equal to the width (diameter) of thebit line selector side pillars of the “U”-shaped strings, and dimensiond in the direction x substantially equal to the width (in the directionx) of a pair of adjacent bit lines. The bit lines BL0-BL7 contact thebit line contact plates P-BLC through the bit line contacts BLC,arranged in an eccentric position, offset in the direction x withrespect to the axes of the bit line selector side pillars of the“U”-shaped strings; the bit line contact plates P-BLC contact the bitline selector side pillars of the “U”-shaped strings by means of thepillar contacts PC.

The arrangement of the 16 “U”-shaped strings in each of the two groupsof “U”-shaped strings differs from that of the first two embodiments. Inparticular, in the generic alignment along the direction y of “U”-shapedstrings of the generic group of 16 “U”-shaped strings, moving from thecenter line of the memory block to the outside along the direction y,the more internal “U”-shaped strings (i.e., whose bit line selector sidepillar is closer to the centerline of the memory block; these are forexample the “U”-shaped strings visible in FIG. 39 whose lower tubularportion is indicated with USHL0 b and USHR1 b) have a first width and afirst depth; “U”-shaped strings immediately following (for example, inFIG. 39 these are the “U”-shaped strings whose lower tubular portion isindicated with USHL1 b and USHR1 b) have a second width and a seconddepth, greater than the first width and the first depth, respectively;“U”-shaped strings still successive (for example, in FIG. 39 these arethe “U”-shaped strings whose lower tubular portion is indicated withUSHL0 a and USHR1 a) have a third width and a third depth, greater thanthe second width and second depth, respectively; more external“U”-shaped strings of the memory block (for example, in FIG. 39 theseare the “U”-shaped strings whose lower tubular portion is indicated withUSHL1 a and USHR0 a) have a fourth width and a fourth depth, greaterthan the third width and third depth, respectively.

The memory block architecture in accordance with the third embodiment isslightly less compact than the architecture in accordance with thesecond embodiment.

Fourth Embodiment

A fourth exemplary embodiment of the invention is shown in FIGS. 45-56.As before, the figures (except for FIG. 53, which shows the equivalentelectrical circuit of a memory with 48 layers) illustrate for simplicitya memory with 4 layers, but this is not to be construed as limiting. Inparticular, FIGS. 45-56 relate to a memory (cell) block, i.e., the setof memory cells that share the same set of (eight in the exampleconsidered) control gates CG0-CG7.

The fourth embodiment can be considered a variation of the thirdembodiment.

Differently from the third embodiment, the bit line selectors arefurther subdivided into sub-groups: with reference to the example shownin the figures, the bit line selectors group BLS0′ is divided into twosub-groups of bit line selectors BLS0″ and BLS1″, the bit line selectorsgroup BLS1′ is divided into two sub-groups of bit line selectors BLS2″and BLS3″, the bit line selectors group BLS2′ is divided into twosub-groups of bit line selectors BLS4″ and BLS5″, and the group of bitline selectors BLS3′ is divided into two sub-groups of bit lineselectors BLS6″ and BLS7″, for a total of eight sub-groups of bit lineselectors. All the bit line selectors of the sub-group BLS0″, BLS1″,BLS2″, BLS3″, BLS4″, BLS5″, BLS6″, BLS7″ are controlled by a same bitline selection control signal #BLS0″, #BLS1″, #BLS2″, #BLS3″, #BLS4″,#BLS5″, #BLS6″, #BLS7″, respectively.

From the constructive point of view, in addition to the two trenches(slits) T and the two additional trenches (slits) T1 already providedfor in the second and in the third embodiments, four yet furtherseparations (slits) T2 and T3 are provided, extending in depth down tothe higher control gate layer, to separate, electrically and physically,the bit line selectors group BLS0″ from the bit line selectors groupBLS1″, and the bit line selectors group BLS6″ from the bit lineselectors group BLS7″.

Having divided the bit line selectors into eight groups, rather thaninto four groups as in the third embodiment, the number of bit lines canbe further halved (as the bit line selection signals are doubled): foran equal number of “U”-shaped strings of the memory block, in the fourthembodiment there are provided 4 bit lines BL0-BL3 instead of the 8 bitlines of the third embodiment.

Thanks to the halving of the number of bit lines, in the fourthembodiment it is possible to avoid having to provide the offset of thebit line contacts to the bit line side pillars of the “U”-shapedstrings. In fact, having halved the number of bit lines, on top of eachalignment of “U”-shaped strings along the direction y now a single bitline of the 4 bit lines BL0-BL3 extends.

The arrangement of the 16 “U”-shaped strings in each of the two groupsof “U”-shaped strings does not change with respect to the thirdembodiment.

In FIGS. 45-56 the lower tubular portions of the 16 “U”-shaped stringsof the group of “U”-shaped strings on the left are indicated withUSHLik, with i=0÷3 and k=a, b, c, d; the index k identifies the group ofbit line selectors BLS0″, BLS1″, BLS2″, BLS3″ which pertains to a given“U”-shaped string, while the index i identifies the row (among the 4rows) in the direction y where the considered “U”-shaped string islocated. The same notation is adopted to indicate the lower tubularportions USHRik of the 16 “U”-shaped strings of the group of “U”-shapedstrings on the right, in which case the index k identifies the bit lineselectors group BLS4″, BLS5″, BLS6″, BLS7″.

The memory block architecture in accordance with the fourth embodimentis slightly less compact than the architecture in accordance with thethird embodiment, as a consequence of the doubling of the number of bitline selectors groups (which requires to provide eight trenches, slitsT, T1, T2 and T3).

Fifth Embodiment

A fifth exemplary embodiment of the invention is shown in FIGS. 57-68.As before, the figures (except for FIG. 65, which shows the equivalentelectrical circuit of a memory with 48 layers) illustrate for simplicitya memory with 4 layers, but this is not to be construed as limiting. Inparticular, FIGS. 57-68 relate to a memory (cell) block, i.e., the setof memory cells that share the same set of (eight in the exampleconsidered) control gates CG0-CG7.

In the above-described embodiments, memory blocks comprising (two)groups of 16 “U”-shaped strings have always been considered. The numberof “U”-shaped strings not to be construed as limiting for the presentinvention, and the fifth embodiment has a memory block architecturesimilar to that of the fourth embodiment but in an example of a memoryblock comprising 2 groups of 12 “U”-shaped strings.

Adopting a notation similar to that of the fourth embodiment, in FIGS.57-68 the lower tubular segments of the 12 “U”-shaped strings of thegroup of “U”-shaped strings on the left are indicated with USHLik, withi=0÷3 and k=a, b, c; the index k identifies the group of bit lineselectors BLS0″, BLS1″, BLS2″ which pertains to a given “U”-shapedstring, while the index i identifies the row (among the 4 rows) in thedirection y in which there is the considered “U”-shaped string. The samenotation is adopted to indicate the lower tubular segments USHRik of the12 “U”-shaped string of the group of “U”-shaped strings on the right, inwhich case the index k identifies the bit line selectors group BLS4″,BLS3″, BLS4″, BLS5″.

In the memory block architecture according to the fifth embodiment sixslits T, T1 and T2 are provided for.

More generally, it is possible to realize memory blocks containing ageneric number n of “U”-shaped strings of memory cells (as well as ageneric number of layers), starting from the previously describedembodiments.

Replicating in the direction x and in direction y one of the memoryblocks according to the embodiments described above, a matrix of memory(cells) is obtained.

The described memory block architectures according to the presentinvention allow to obtain very compact structures, and consequentlymemories with high data storage capacity per unit area.

What is claimed is:
 1. A 3D memory device comprising: a substrate; atleast one first group of at least three first U-shaped memory cellsstrings each including a first buried string portion, a first sourceline selector side string portion and a first bit line selector sidestring portion, wherein the first buried string portion is formed in thesubstrate and connects the first source line selector side stringportion and the first bit line selector side string portion, each of thefirst U-shaped memory cells strings including memory cells stacks alongthe first source line selector side string portion and along the firstbit line selector side string portion; and at least one second group ofat least three second U-shaped memory cells strings each including asecond buried string portion, a second source line selector side stringportion and a second bit line selector side string portion, wherein thesecond buried string portion is formed in the substrate and connects thesecond source line selector side string portion and the second bit lineselector side string portion, each of the second U-shaped memory cellsstrings including memory cells stacks along the second source lineselector side string portion and along the second bit line selector sidestring portion, wherein the first and second source line selector sidestring portions are between the first and second bit line selector sidestring portions, and wherein the at least three first U-shaped memorycells strings are mutually co-planar and one surrounded by the other,and the at least three second U-shaped memory cells strings are mutuallyco-planar and one surrounded by the other.
 2. The 3D memory device ofclaim 1, wherein the at least three first U-shaped memory cells stringshave the respective first buried string portions formed at a first, asecond and a third different depths in the substrate and with a first, asecond and a third different lengths, the at least three second U-shapedmemory cells strings have the respective second buried string portionsformed at the first, the second and the third depths in the substrateand with the first, the second and the third lengths.
 3. The 3D memorydevice of claim 2, wherein the length of the first and second buriedstring portions increases with their depth in the substrate, andwherein: the first depth is less than the second depth and the seconddepth is less than the third depth, and the first length is less thanthe second length and the second length is less than the third length.4. The 3D memory device of claim 1, wherein the at least three firstU-shaped memory cells strings are co-planar with the at least threesecond U-shaped memory cells strings.
 5. The 3D memory device of claim1, further comprising: a plurality of source line selector side wordlines stacked over the substrate, wherein the plurality of source lineselector side word lines surrounds the first source line selector sidestring portions and the second source line selector side stringportions; and a first plurality and a second plurality of bit lineselector side word lines stacked over the substrate, wherein the firstplurality of bit line selector side word lines surrounds the first bitline selector side string portions and the second plurality of bit lineselector side word lines surrounds the second bit line selector sidestring portions.
 6. The 3D memory device of claim 5, further comprising:source line selectors stacked over the plurality of source line selectorside word lines, wherein the source line selectors surround the firstsource line selector side string portions and the second source lineselector side line string portions, wherein the source line selectorscomprise one source line selector for each of the first and secondsource line selector side string portions; and first bit line selectorsstacked over the first plurality of bit line selector side word linesand second bit line selectors stacked over the second plurality of bitline selector side word lines, wherein the first bit line selectorssurround the first bit line selector side string portions and the secondbit line selectors surround the second bit line selector side stringportions, and wherein the first bit line selectors comprise one firstbit line selector for each of the first bit line selector side stringportions and the second bit line selectors comprise one second bit lineselector for each of the second bit line selector side string portions.7. The 3D memory device of claim 6, wherein the plurality of source lineselector side word lines comprises a stack of layers of source lineselector side word lines, the source line selector side word lines ineach layer being electrically connected, and the source line selectorsare formed so as to be controlled by a common single source lineselector control signal.
 8. The 3D memory device of claim 6, wherein thefirst bit line selectors are formed so as to be controlled by a commonsingle first bit line selector control signal, and the second bit lineselectors are formed so as to be controlled by a common single secondbit line selector control signal.
 9. The 3D memory device of claim 6,comprising at least a first bit line operatively associated to at leasttwo of the at least three first and second U-shaped memory cellsstrings, wherein the at least a first bit line is connected, through arespective first bit line selector and second bit line selector, to thefirst bit line selector side string portions of the at least two of theat least three first U-shaped memory cells strings and to the second bitline selector side string portion of the at least two of the at leastthree second U-shaped memory cells strings.
 10. The 3D non-volatilememory device of claim 1, wherein center lines of the first buriedstring portions are located in a same respective line, and center linesof the second buried string portions are located in a same respectiveline.